Field of the Invention
The present invention relates to a method of attenuating the blast overpressure caused by the escaping gases whenever a projectile exits a weapon tube—which overpressure is known to cause temporary and permanent hearing loss to the weapon's user.
Related Art
Blast overpressure (BOP) is a phenomenon that is encountered when a blast wave is formed due to release of a relatively high amount of energy into the surrounding environment, for example, when a projectile is expelled from the muzzle of a hand gun, a mortar, or a large caliber weapon, such as a howitzer. Such a high energy blast wave is in-part manifested as an impulse of noise, a sound wave—which expands about the rear of the muzzle to impact the weapon's user or crew, causing both temporary and permanent hearing loss.
The ear damage and resulting hearing loss caused by a blast wave/blast overpressure from a weapon discharge is directly related to the peak pressure level of the sound wave experienced by the weapon's user—though other factors, such as sound frequency, energy spectrum, rise time of the sound wave, duration of the sound wave, repetition rate and total number of exposures to the sound wave are all relevant. Peak pressure is defined quantitatively in psi (pounds per square inch) or dB (decibels) above a base or reference level (i.e. the auditory threshold of 0.0002 dynes per square centimeter or 20 μPa). Such hearing damage can result from exposure to a sound pressure level over the auditory threshold of about 85 dB, even if the exposure isn't continuous; which level is significantly below the threshold of pain, which is understood to be about 130 dB. In comparison, the peak sound pressure level, over the 20 μPa auditory threshold for an M16 rifle at the gunner's left ear is about 154 dB, and the corresponding peak sound pressure level for a 105 mm towed howitzer is from about 185 dB.
Various techniques have been attempted to mitigate the BOP effect on the crew serviced military weapons, especially cannons and mortars—where the BOP and sound pressure effects are significantly greater than with a hand weapon. Such techniques include—(1) reducing the propellant—which unfortunately also reduces the range of the weapon; (2) using muzzle devices to cool, expand, diffuse and expel the muzzle gas flow, such as a double baffle sound-suppressor for the M16, which can reduce the peak sound level to about 148 dB (a reduction of 6 dB)—but, which adds weight, cost, and complexity to the weapon; (3) using another type of muzzle device known as a Blast Attenuation Device (BAD), which is commonly used on mortars to funnel and accelerate the blast in the direction of the projectile, i.e. deflecting it away from the motor's crew—but, which again adds weight and adds significant length to the mortar tube (a significant problem for loading longer tube mortars (e.g. 120 mm mortars); especially when such mortars were ground mounted—cases where shorter crew members have to stretch to hang the rounds even without the BAD device's added length).
A large number of investigations have been performed on the propagation of blast wave resulting from a variety of cannon systems. These studies include both experimental and numerical work—including for example, Schmidt et al, “Analysis of Weapon Parameters Controlling the Muzzle Blast Overpressure Field,” 5th International Symposium on Ballistics, 1980, where a series of experiments were conducted on a 20 mm cannon to study weapon exhaust properties, near flow-field structure, and the blast overpressure. Schmidt found that it is important to account for the precursor/propellant gas interactions in the description of the near muzzle flowfield. Schmidt developed a closed-form expression for the overpressure around the weapon muzzle that fits with a variety of cannon systems. Similarly, Klingenberg, “Investigation of Combustion Phenomena Associated with the Flow of Hot Propellant Gases—III: Experimental Survey of the Formation and Decay of Muzzle Flowfields and Pressure Measurements,” Combustion and Home, Vol. 29, No. 3, 1977, pp. 289-309, also looked into aspects of muzzle blast for a 7.62 mm rifle through shadowgraph examined the muzzle blast field. While providing an increased understanding of the physical events occurring within and about the muzzle blast field and overpressure—these studies did not propose alternative weapon designs versus the current state of the art—to mitigate overpressure and its effects.
In parallel with the experimental work discussed above, various purely numerical studies have also been carried out providing further insight into blast wave structure and propagation. For example, Erdos and Del Guidice, Calculation of Muzzle Blast Flowfields,” AIAA Journal, Vol. 13, No. 8, 1975, pp. 1048-1055, disclosed numerical simulations of the unsteady shock layer bounded by the mach disk and blast wave. More recently, numerical studies have been conducted on blast mitigation utilizing a muzzle device; wherein, for example, Kang et al, “A Study on Impulsive Attenuation for High-Pressure Blast Flowfield,” Journal of Mechanical Science and Technology, Vol. 22, 2008, pp. 190-200, carried out numerical analysis of high-pressure supersonic blast flowfields and compared the results to several experiments; which results, indicated optimum design considerations for silencers. Additionally, Rehman et al, “Analysis and Attenuation of Impulsive Sound Pressure in a Large Caliber Weapon During Muzzle Blast,” Journal of Mechanical Science and Technology, Vol. 25, 2011, pp. 2601-2606, showed blast attenuation on a 120 mm cannon utilizing a three-baffled silencer. However, none of these studies ended provided any alternative weapon designs versus the current state of the art—to mitigate overpressure and its effects.
Thus there is a need in the art for a means to attenuate the blast wave/blast overpressure that results from a weapon discharge, such that the peak pressure level/sound wave which impacts the user or crew of the weapon is significantly reduced, to mitigate the harm caused to the user or crew; and, where the method does not significantly reduce the range of the weapon, add to its weight, or otherwise detract from its serviceability—as does prior art solutions.